A present invention relates to an excellently reliable force sensor which can three-dimensionally detect the magnitude and the direction of a physical quantity by detecting displacement of an operating member or a strain caused in the flexible plate by the displacement of the operating member and aims to protect the flexible plate from being damaged / destroyed by the action of impulsive force applied suddenly or serious external force without deterioration of detection sensitivity.
There has been increasing a demand for sensors capable of accurately detecting a physical quantity such as acceleration, magnetism, or the like, in fields of an automobile and a mechanical industries. Particularly, there is a need for a small-sized sensor capable of detecting such a physical quantity for each of two- or three-dimensional components. For example, an acceleration sensor is used for a mechanism for automatically recovering a balance when an automobile lost its balance due to sudden wheeling, a side wind, or the like, a collision sensing mechanism, a mechanism for adjusting self-supporting stability of a crane, or the like, a mechanism for adjusting a flow rate of fluid or for opening and shutting a valve by sensing a change in a speed of fluid flowing through pipe, etc.
As such a sensor, Japanese Patent Laid-Open 5-26744 discloses a sensor 95 in which a plurality of piezoelectric element 98 are disposed on a flexible plate 97 suspending a weight 96 as shown in FIG. 15. In addition, as shown in FIG. 14, Japanese Patent Laid-Open 8-94661 discloses a sensor 90 in which a supporting base 98, a flexible plate 92, and a weight 94 disposed inside the sensor are unitarily formed using a piezoelectric material, one end surface of a cylindrical supporting base 93 is blocked up by the flexible plate 92, the columnar weight 94 is suspended by the flexible plate 92 at the center of a hollow portion of the supporting base 93, a plurality of upper electrodes 91A-91E are disposed on the surface of the flexible plate 92, and a lower electrode 91F is disposed on the lower surfaces of the supporting base 93, the flexible plate 92, and the weight 94.
These sensors are constituted so that the flexible plates are bent by a force corresponding to a physical quantity such as a direct force acting on the weight from outside, a force of inertia due to an acceleration and a magnetic attraction. The sensors can detect the magnitude and the direction of a physical quantity by detecting an electric charge generated in a piezoelectric body in accordance with bending of the flexible plate. Such a sensor is hereinbelow referred to as xe2x80x9ca force sensor xe2x80x9d.
In three-dimensional detection of a force, that is, in detection of a force in X, Y, and Z axial directions (hereinafter referred to as three axial directions) which shows rectangular coordinates, for example, in a sensor shown in FIG. 14, a displacement can be detected by a charge generated in the upper electrode 91E in the case that the weight 94 is displaced in the z-axial direction which is a direction of suspension of the weight 94. At this time, force in the X-axial direction and in the Y-axial direction is prevented from being detected by wiring so that the charges generated in the upper electrodes 91A and 91B offset each other and the charges generated in the upper electrodes 91C and 91D offset each other. In the same manner, a displacement in each of the directions can be measured by charges generated in the upper electrodes 91A and 91B when the weight 94 is displaced in the X-axial direction and charges generated in the upper electrodes 91C and 91D when the weight 94 is displaced in the Y-axial direction. Thus, when the weight 94 is displaced in any direction, the magnitude and the direction of an applied physical quantity can be known by synthesizes detected components in each axial direction.
The aforementioned force sensor uses a stress generated in the flexible plate by a displacement of the weight. When the stress exceeds the fracture strength of the flexible plate, a crack is caused in the flexible plate and results in breakage. For example, when a sensor is dropped and collides against the ground due to a mistake, or the like, an excessive acceleration is applied to the weight, which cause a crack in a boundary portion between the weight and the flexible plate. Thus, breakage of a sensor is sometimes caused.
In addition, when fatigue of the flexible plate advances due to a long-term use, breakage is sometimes caused even if the stress generated in the flexible plate does not reach the material fracture strength. This is because a kind, a shape, or the like, of a material for each member has conventionally been set up so that a desired detection sensitivity is obtained without breakage of the flexible plate when the maximum acceleration to be detected is given to the sensor.
By thickening the flexible plate to increase the strength, it is possible to avoid the breakage with a certain probability even if, for example, the aforementioned sudden serious external force acts. However, the flexible plate becomes difficult to bent in this case, and therefore sensitivity of the sensor is lowered.
For example, FIG. 16(a) shows an example of a result that a stress generated in the flexible plate 71 when an operating member 72 is displaced with 1G acceleration applied to the acceleration sensor 70 having a flexible plate 71 comprising a piezoelectric body, an operating member 72, a supporting base 73, and a structure of the same rank as that of the acceleration sensor 90 shown in FIG. 14 in Z-axial direction is obtained by FEM simulation. Here, there were used, as a parameter, a supporting base 73 made of zirconia and having an inner diameter of 3.5 mm, an operating member 72 made of zirconia and having a diameter of 1.8 mm and a thickness of 0.635 mm, and a flexible plate 71 having a zirconia plate having a thickness of 0.015 mm and a PZT element having a thickness of 0.02 mm superposed on the zirconia plate to have a film form. Physical properties of the zirconia and the PZT element used for the simulation are shown in Table 1.
As shown by a curved line(stress line) showing a stress in FIG. 16(a), when an acceleration is given in the Z-axial direction, a large stress force is generated in the X-axial and Y-axial directions, and the stress line has a peak. This shows that the stress is concentrated in a narrow range of length (a transversal axis). Therefore, an acceleration sensor has conventionally designed so as to avoid breakage by adjusting thickness of a flexible plate as a whole so as not to exceed limit of breakage of the flexible plate by; for example, supposing that the maximum stress is excessive stress upon collision with the ground. However, this method cause a problem of deterioration in sensitivity of the sensor because a mode of generating stress is not changed, and therefore, if a peak value of the stress is set to be the same as or lower than the limit of breakage, whole magnitude of stress to be generated becomes small.
This is hereinbelow explained from another view point. The whole magnitude of stress shows a detecting sensitivity of a sensor, and an area surrounded by a stress curve and a transverse axis showing a length of a flexible plate expresses a detection sensitivity. Therefore, as shown in FIG. 16(b), a mode of generation of overall stress, that is, only a peak value becomes low without changing an outline having a peak shape, an area surrounded by the stress curve and the transverse axis becomes small, and a detecting sensitivity is lowered.
In a force sensor, it is important to improve productivity besides a problem of balance of the aforementioned sensitivity and reliability. The weight and the supporting base are required to have high rigidity to hardly bend to purely detect an acceleration, while the flexible plate is required to have high flexibility to obtain sufficient sensitivity. To satisfy these properties, it is possible to produce by assembling each of the weight, the supporting base, and the flexible plate. However, it is difficult to desire improvement in productivity because it requires many parts and processes.
The present invention has been made in view of the aforementioned problems of prior art and aims to obtain a force sensor which is excellent in reliability and productivity and which can obtain a desired detecting sensitivity with lowering a peak value of stress and without reducing the whole magnitude of stress generated in the flexible plate.
According to the present invention, there is provided a force sensor comprising:
a supporting base having a hollow portion,
a flexible plate having at least one detecting element and extending across over the hollow portion, and
an operating member suspended over the hollow portion by the flexible plate;
wherein inclined strength is given to the flexible plate so that it becomes stronger from an arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the outer periphery of the operating member and/or toward the inner periphery of the supporting base.
It is preferable that the inclined strength is given to the flexible plate by giving an inclined thickness to the flexible plate so that it becomes thicker from an arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the outer periphery of the operating member and/or toward the inner periphery of the supporting base.
A force sensor of the present invention includes a force sensor capable of detecting a force in only one-axial (linear) direction, a force sensor capable of detecting a force in arbitrary two-axial (plane) directions, and a force sensor capable of detecting a force in three-axial directions. A detecting direction by these force sensors can be selected by changing conditions of disposition or wiring of a detecting element.
In a force sensor of the present invention, there is preferably employed a method in which the inclined strength is given to the flexible plate by giving an inclined thickness to the flexible plate so that it becomes thicker from an arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the outer periphery of the operating member and/or toward the inner periphery of the supporting base. A boundary between the flexible plate and the operating member and/or a boundary between the flexible plate and the supporting base preferably have(has) a curvature. The curvature of the boundary between the flexible plate and the operating member may be different from that of the boundary between the flexible plate and the supporting base.
A gradient of the thickness of the flexible plate from the arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the outer periphery of the operating member are preferably different from that of the flexible plate from the arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the inner periphery of the supporting base. Though the gradient of the thickness is preferably continuous, the flexible plate may be formed to have the inclined thickness arranged in tiers. When the flexible plate may be formed to have the inclined thickness arranged in tiers, it is possible and preferable to employ a green sheet lamination technique.
It is also preferable that the inclined strength is given to the flexible plate by giving an inclined composition to the flexible plate from an arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the outer periphery of the operating member and/or toward the inner periphery of the supporting base. In this case, the inclined strength is preferably given to the flexible plate by giving an inclined composition to the flexible plate from an arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward both the outer periphery of the operating member and the inner periphery of the supporting base. A condition of the composite of the flexible plate from the arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the outer periphery of the operating member may be different from that of the flexible plate from the arbitrary position between the outer periphery of the operating member and the inner periphery of the supporting base toward the inner periphery of the supporting base. Of course, inclined thickness and composition may be employed simultaneously.
According to the present invention, there is further provided a force sensor comprising:
a supporting base having a hollow portion,
a flexible plate having at least one detecting element and extending across over the hollow portion, and
an operating member suspended over the hollow portion by the flexible plate;
wherein a boundary between the flexible plate and the operating member and/or a boundary between the flexible plate and the supporting base have(has) a curvature.
In such a force sensor it is preferable that both a boundary between the flexible plate and the operating member and a boundary between the flexible plate and the supporting base have a curvature. It is also preferable that the curvature of the boundary between the flexible plate and the operating member is different from that of the boundary between the flexible plate and the supporting base. The boundary may be formed in tiers. In all the aforementioned force sensors of the present invention, a piezoelectric element is suitably used as the detecting element. The operating member, the supporting base, and the flexible plate are unitarily formed, and at least one of them has a different chemical composition from the others. By making only the flexible plate of a zirconia ceramic containing 0.1-0.6% by weight of titanium in terms of TiO2 and/or 0.005-0.1% by weight of magnesium in terms of MgO, goof flexibility can be preferably obtained. By making only the flexible plate of a zirconia ceramic containing 0.2-0.5% by weight of titanium in terms of TiO2 and/or 0.01-0.05% by weight of magnesium in terms of MgO, the better flexibility can be preferably obtained.
A hardness of a part of the detecting element formed on a part of or an entire surface of the flexible surface is preferably made higher in comparison with that of the other part by a hardening material so as to partially heighten strength of the flexible plate.